Low-power ethernet transmitter

ABSTRACT

An electrical circuit comprising a line driver for providing Ethernet signals is disclosed. The line driver comprises a voltage mode line driver for producing 1000BT and 100BT Ethernet signals and an active output impedance line driver arranged parallel to the voltage mode line driver. The line driver is capable of producing 1000BT or 100BT or 10BT Ethernet signals, wherein either the voltage mode line driver or the active impedance line driver is active.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 11/798,334, filed on May 11, 2007, which claims benefit to U.S. Provisional Application No. 60/799,346, filed on May 11, 2006, both of which are incorporated by reference in their entirety herein.

BACKGROUND OF THE INVENTION

The Institute of Electrical and Electronics Engineers (IEEE) inter alia sets the standards for communication devices interchanging information using the Ethernet protocol in order to enable for example different manufacturers to produce devices complying with the same specifications and thus being compatible to each other. For example 10BT is a well known Ethernet standard protocols for transmitting digital information at a transmission speed of 10 Mbit/s, 100BT defines digital data transmission at 100 Mbit/s and 1000BT defines the transmission at 1000 Mbit/s=1 Gbit/s.

The IEEE 802.3 standard defines the requirements for the combined 10BT/100BT/100BT transmitters using unshielded twisted pair (UTP) lines. That is IEEE 802.3 for example defines what voltage levels should be output on the lines, how the switching between the different voltage levels defined for the protocols should be handled and what termination impedance should be guaranteed on the line.

For the transmission protocols different requirements are specified in the standard. For example the highest transmitter linearity is required in 1000BT in the presence of an interferer put on the line in full duplex. The highest accuracy of signals is required in 100BT mode when fast settling with accurate rise time and accurate output voltage are specified. The 10BT protocol requires the highest voltage swing.

Conventional solutions for a combined transmitter being able to output signals according to all three protocols use a single complex current-mode digital-to-analog converter (DAC) to shape the edges of the 100BT and 1000BT waveforms and provide a very accurate equalization in 10BT mode. It is known that meeting the defined requirements of transmission speed, signal accuracy, linearity and output voltage swing with the same transmitter proves costly in terms of power consumption. To meet the 100BT/1000BT amplitude requirements and to keep the proper line termination, for example by means of a resistor parallel to the driver the current-mode DAC need a full-scale current of 40 mA. If the driver is operated in 10BT mode the same current-mode DAC needs a full-scale current of 100 mA to meet the specifications of this protocol, which specifies larger signal amplitude, namely 5V peak-to-peak, for 10BT. The specification of the large amplitude in 10BT furthermore requires a common voltage at the DAC output of at least 1.8 Volts. The power consumption of the transmitter thus results in 72 mW when operating in 100BT or 1000BT mode and in 180 mW in 10BT mode. Furthermore the list of materials needed for the final assembled product is increased by a low dropout regulator (LDO) located off the chip, which supplies a center-tap current for the current-steering DAC.

Following from the afore mentioned specifications there is a need for a transmitter capable of producing output signals according to the 10BT and the 100BT and the 1000BT specifications with low power consumption on a small silicon area.

SUMMARY OF THE INVENTION

The invention relates to an electrical circuit comprising a line driver for providing Ethernet signals, the line driver comprising a voltage mode line driver parallel to an active output impedance line driver.

Furthermore a method is disclosed for producing an Ethernet signal from an analog signal reflecting the symbols to be transmitted, wherein the analog signal is fed into a voltage mode line driver, and wherein the analog signal is fed into an active impedance line driver.

DESCRIPTION OF THE FIGURES

The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the present invention and together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention.

FIG. 1 depicts an overview of the driver circuit

FIG. 2 illustrates an embodiment of a voltage mode line driver

FIG. 3 depicts a circuit for a single ended amplifier

FIG. 4, a) depicts a circuit for a differential amplifier, b) depicts a circuit for producing a calibration voltage for the differential amplifier

FIG. 5 depicts a first implementation of an active impedance output driver circuit

FIG. 6 depicts an alternative implementation of an active impedance output driver circuit.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described in detail with reference to a few preferred embodiments thereof as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known processes and steps have not been described in detail in order not to unnecessarily obscure the present invention.

FIG. 1 illustrates a schematic of the invention. Elements or processing blocks in circuit 100, which generate the symbols to be transmitted are omitted in the drawing. Only the last elements in the processing chain of a chip are illustrated, which process the information to be signaled over a line before the signals leave the chip at the connection pads 110 of the chip. The connection pads may be coupled to a cable or line according to the standards as defined in the IEEE 802.3 standard.

Digital symbols to be transmitted are fed into a digital-to-analog converter (DAC) 120, which at the same time receives suitable clock signals corresponding to the transmission speed of the signal output at the pads 110. DAC 120 outputs analog signals corresponding to its digital input signals, i.e. the digital symbols. Depending on the transmission speed at which the signal will be output at pads 110 the analog signal is processed by either the voltage mode line driver 130 or the active impedance line driver 140. The analog signal accordingly is multiplexed to either of the line drivers by two current-steered FET transistors, shown as multiplexer 170 in the drawing.

If the analog signal output of DAC 120 is to be sent as 100BT or 1000BT signals, then the voltage mode line driver 130 will be active and will process the incoming signal to produce corresponding 100BT or 1000BT signals respectively. The active output impedance line driver 140 is inactive while the voltage mode line driver 130 processes the signal to leave the 100BT or 1000BT signal generated by the voltage mode line driver unaffected.

The signals output by the voltage mode line driver pass the Rx block 150 unmodified and are coupled to the pads of the chip.

As the 1000BT protocol is full duplex, i.e. signals can be sent to and received from a remote station at the same time and on the same line, the path from the voltage mode line driver to the pads includes Rx block 150, which filters incoming signals, i.e. Rx signals and forwards them for further processing.

In case that the symbols are to be sent as 10BT signals, then DAC 120 will be clocked by an appropriate clock signal and output a corresponding analog signal, which will be processed by the active output impedance line driver 140 outputting a corresponding 10BT signal. The voltage mode line driver 130 is inactive in this case, i.e. does not produce any output in order to keep the 10BT signal unaffected.

In this way the analog signal output by DAC 120 is processed by one of the two line drivers, while the second line driver is inactive, i.e. does not produce any voltage at its respective output and provides a very high output impedance, i.e. much higher than the impedance of the transmission line coupled to the pads, to leave the signal generated by the first driver unaffected.

The processing of the symbols using the circuit depicted in FIG. 1 comprises method steps according to the elements shown. In a first step the symbols to be transmitted for example as a signal as defined in the IEEE Ethernet standard is fed into a digital-to-analog converter 120, which outputs an analog signal corresponding to the input symbols. Depending on the speed of the signal to be produced the analog signal is then fed either to the voltage mode line driver 130 for example for generating a 100BT or a 1000BT Ethernet signal or to the active output impedance line driver for generating a signal according to the IEEE 10BT Ethernet specification. Accordingly and for preventing a mix up of generated signals either the voltage mode line driver 130 or the active output impedance line driver 140 processes the analog signal output from DAC 120. The Ethernet signal output from either of the line drivers is coupled to termination pads of the chip carrying circuit 100.

Typically circuit 100 and the corresponding signal processing method can be used in a transmission system changing transmission speeds during a data transfer. For example an electrical circuit comprising circuit 100 can be initially powered up in 10BT mode. In the initial negotiation with a remote transceiver the communication may start using 10BT mode, wherein in circuit 100 the analog signal output from DAC 120 is fed to the active output impedance line driver. If the remote transceiver is capable of processing 100BT Ethernet or 1000BT Ethernet signals, then one of these higher speed protocols may be negotiated. Circuit 100 accordingly switches the transmission speed by feeding the signals output from DAC 120 to the voltage mode line driver 130 to produce signals according to the protocol agreed between the transceivers.

In a variation of the circuit depicted in FIG. 1 DAC 120 can be a 17-level DAC followed by a low-pass filter 160, which can be implemented in the feedback paths of the amplifiers comprised in the voltage mode line driver and in the active output impedance line driver.

For 1000BT mode a PAM-5 (5 level) coding scheme is followed by a first-order partial response filter that increases the number of levels to 17, as required by the IEEE specification. Thus the resolution of the 17-level DAC matches the specification. Each symbol is generated by a single step output from the DAC every 8 ns. The low-pass filter is configured to have its single pole at 94 MHz to correctly shape the output signal as given in the IEEE specifications. To ensure this pole does not change with process variations a calibration circuit locks the pole at 94 MHz to an accurate time reference, which in this case is a 125 MHz clock.

The resulting 1000BT output waveform has a rise time of 3.7 ns and passes all templates and required IEEE specifications.

For 100BT mode a MLT-3 (3 level) coding scheme is given in the specification. A first intermediate step is made to 50% of the final amplitude, and an final step is made 2 ns later to 100% of the final amplitude. In this case the final step is added 2 ns after the start of the symbol time, which is 8 ns. Only 5 of the 17 available DAC levels are therefore used. The low-pass filter is configured to have its single pole at around 141 MHz to correctly shape the output signal. An additional clock signal is used to start the final step at 2 ns. Note that the pole frequency is not critical for 100BT mode. Also note that other, similar combinations of multi-step DAC outputs, separated by a similar time delay, followed by single or multiple pole filtering can meet the requirements.

The resulting 100BT output waveform has a rise time of 3.7 ns, settles to 1% of its final value in 6.5 ns.

For 10BT mode an equalizer preceding the DAC creates a shaped pulse based on a raised cosine. This pulse is quantized to 17 levels and fed to the DAC. The DAC is clocked at 160 MHz and updates its output every 6.25 ns to create the full symbol lasting 100 ns; therefore, 16 DAC samples make up one 10BT symbol. The low-pass filter is configured to have its single pole at around 80 MHz to correctly shape the output signal. Note that the pole frequency is not critical for 10BT mode and that clocking rates other than 160 MHz are suitable.

The resulting 10BT output waveform contains its highest unwanted harmonic at −25 dB below the carrier and correctly passes all templates and required IEEE specifications.

In this way the combination of the DAC followed by the low-pass filter allows the use of a low-complexity DAC comprising only 17 levels. A single-pole low-pass filter can be implemented easily using only passive components, thus saving power. These two analog circuits together can be configured to meet all IEEE specifications for all 3 modes of operation without having to add additional circuitry.

As mentioned above the single-pole low-pass filters can be implemented in the feedback paths of the amplifiers comprised in the line drivers. Accordingly the feedback path of the voltage mode line driver implements an adjustable single-pole low-pass filter, which can be adjusted to a pole frequency of around 94 MHz for processing 1000BT Ethernet signals and to a pole frequency of around 141 MHz for processing 100BT Ethernet signals.

Circuit 200 as depicted in FIG. 2 exemplifies a voltage mode line driver 130. It is to be noted that other topologies of voltage mode line drivers can be used, as long as these are line drivers of the voltage mode type, i.e. the output impedance Zout of the line driver is zero.

As indicated by the arrow circuit 200 is coupled to a digital-to-analog converter, which is DAC 130 as shown in FIG. 1. The DAC provides a current Isig as input signal to circuit 200. In case that the DAC provides a voltage mode output, then circuit 200 is coupled by a conventional voltage-to-current converter.

Voltage mode line driver 200 consists of a pseudo-differential transimpedance amplifier. The symbol dependent input current Isig is coupled to the negative input terminals of a first and a second operational amplifier 210 and 211. The positive input terminals of the amplifiers are coupled to a common mode voltage Vcm.

Input current flows through feedback resistors 220, 221 thus generating the output voltage of the driver specified by the IEEE Ethernet standard and corresponding to the digital symbol to be transmitted.

Capacitors 230, 231 are arranged parallel to the feedback resistors 220, 221 thus forming a 94 MHz low-pass filter limiting the bandwidth of the signal output on the transmission line. In one implementation example the value of a feedback resistor 220, 221 is R=2.1 kΩ and the value of a capacitor 230, 231 is C=800 fF. The feedback path in this way forms the single-pole low-pass filter for filtering 1000BT output signals in case the optional 17-level DAC is used. Furthermore and in case the 17-level DAC is used the value of the capacitor can be adjusted via an electrical signal, such that the frequency of the single-pole low-pass filter can be adjusted to a frequency of around 141 MHz. That is for processing 1000BT signals the feedback path forms a single-pole low-pass filter of around 94 MHz and for processing 100BT of around 141 MHz.

Besides forming a low-pass filter in connection with the parallel resistor the capacitors 230, 231 help to stabilize the operation of operational amplifiers 210, 211, because the feedback capacitors effectuate a zero in the open loop transfer function.

As the amplifiers 210, 211 keep the impedance Zout at their outputs low, i.e. ideally at 0Ω, the termination of the transmission line is achieved by two resistors 240 and 241 in series with the transmission line, each resistor having a value of 50Ω. The output swing in this way is doubled.

The operational amplifiers 210, 211 are designed as class-AB amplifiers to save power. With their bias current being strongly dependent on the symbol to transmit, the power consumption of the amplifiers is very low when no signal is output. Also the power consumption is almost directly proportional with the signal swing of the output signal for large swings. The theoretical power consumption limit for an ideal class-B amplifier is 16.5 mW from a 3.3 Volts supply for a random PAM-5 1000BT signal and 13.2 mW for a PAM-5 signal plus partial response filter transmission. Although power consumption of a class-AB amplifier is higher, it is still about two times lower than conventional solutions.

Circuit 200 comprises resistors 250, 251 and 260, 261 forming a filter for separating signals received on the transmission line when operating in 1000BT mode. For example the values of the resistors can be 2.5 kΩ for resistors 250, 251 and 1.25 kΩ for resistors 260, 261. As 100BT is a half-duplex mode the filter may also be used when receiving signals in 100BT mode. The received, separated signals Rx are then fed to a subsequent processing block—not shown—for further processing.

This exemplifying embodiment of the voltage mode line driver uses two single ended operational amplifiers. In an alternative embodiment a single differential line driver can be used, which substitutes the two single ended amplifiers. In this case the common mode voltage Vcm is arranged in the common mode feedback loop circuit.

FIG. 3 shows a more detailed illustration of an embodiment of one of the two single ended operational amplifiers comprised in voltage mode line driver.

Operational amplifier 300 is class-AB and operated with 3.3 Volts supply voltage as indicated in the figure. The amplifier comprises a transconductance input stage 310 implemented with NMOS transistors 313, 314 connected through the folded cascode stage 330 to the class-AB biased output stage 350.

Stage 310 comprises two appropriately biased cascaded thick oxide NMOS transistors 311 and 312 as a current source for biasing. These allow a fixed (class A) current of 1.5 mA for the input stage. The positive input voltage Vip and the negative input voltage Vin are coupled as indicated to transistors 313 and 314.

The input transconductance stage is coupled to the folded-cascode stage 330 comprising appropriately biased and cascoded PMOS transistors that pass the signal 320 to the output stage through the cascode transistor 331 and an NMOS current mirror to invert the phase of signal 321 generated by the input stage such that the class-AB biasing mesh made of transistors 332,333 receives two in-phase signals at its ends, capable of driving the output stage 340 comprising two transistors 341, 342 connected as inverting amplifiers.

The output stage furthermore comprises a first resistor 345 in series with a miller capacitor 347 and a second resistor 346 in series with a second miller capacitor 348, which improve the stability of the amplifier, and wherein the value of resistors 345, 346 can be 400Ω and the value of capacitors 347, 348 can be 800 femto Farad.

The circuit can be implemented for example in 90 nm CMOS technology, wherein a thin oxide transistor is designed to hold a voltage of 1.2 Volts maximum and a thick oxide transistor is designed to hold at least a voltage of 2.5 Volts.

As mentioned above circuit 200 comprises two single ended amplifiers for example as described in FIG. 3, wherein the positive input of each of the amplifiers is coupled to a common mode voltage of half the supply voltage, which ideally is 1.65 Volts. Accordingly the positive input vip of both single ended amplifiers is coupled to the common mode voltage, whereas the negative input of one amplifier is coupled to the positive output signal from DAC 120 and the positive input of the other amplifier is coupled to the negative output signal from DAC 120. Analogously one of the amplifiers outputs a positive portion Vop and the other amplifier outputs a negative portion of the output signal Von.

Circuit 400 as illustrated in FIG. 4 a exemplifies an embodiment of a differential operational amplifier, which can be used in the voltage mode line driver instead of the two single ended operational amplifiers. Note that different conventional topologies for differential operational amplifiers can be used also.

Differential amplifier 400 is operated at a voltage of ideally 3.3 Volts as indicated in the drawing. The amplifier is symmetrically built up as indicated by dotted mirror line 401, such that processing of signals for generating the negative output voltage Von and the positive output voltage Vop is performed by similar topologies, wherein the coupling to the supply line is inverted between the portions, such that the circuit portion on the left hand side of 401, which outputs the negative output voltage is coupled to the supply voltage of 3.3 Volts where the elements of the right hand portion of the circuit is coupled to ground or a reference potential. Furthermore some of the NMOS transistors have corresponding transistors formed as PMOS transistors in the other portion.

Cascoded thick oxide PMOS transistors 410, 411 provide a supply current to a transconductance input stage 420 comprising transistors 421, 422, to which the positive respectively the negative input signal from DAC 120 are coupled. Similarly cascoded thick oxide NMOS transistors 431, 432 provide a supply current to transconductance input stage 440 comprising transistors 441 and 442, to which the positive and respectively the negative input signals Vip, Vin from DAC 120 are coupled. In fact the gate of transistor 421 is shorted with gate of transistor 441 and the gate of 422 is shorted with the gate of 442. The output signals of the transconductance stages are passed through class-AB stages 450 and 460 to the output stages 470 and 480, wherein output stage 470 provides the negative output voltage Von and output stage 480 the positive output voltage Vop of differential amplifier 400 respectively.

The output stages 470, 480 are of similar topology as those described with reference to FIG. 3. For example stage 470 comprises a cascode of a thin oxide and a thick oxide PMOS transistor 471, 472 and a cascode of a thin oxide and a thick oxide NMOS transistor 473, 474, wherein each cascode has miller capacitor 475, 476 and a resistor 477, 478 in its feedback path.

The output stages and the class-AB stages are furthermore coupled to transistors 490, 491, which are coupled to a voltage vbp_d.

Voltage vbp_d is generated by a common feedback circuit as illustrated in FIG. 4 b, wherein the upper line 4100 is coupled to supply voltage of 3.3 Volts and line 4101 is coupled to ground or any reference potential.

The common feedback circuit comprises a first voltage divider, which for example in this embodiment consists of a first and a second resistor of equal value, for example both being 1 kΩ in value. The center voltage Vcm produced by the voltage divider is half the supply voltage and is tapped as a bias voltage for thick oxide NMOS transistor 4120. A second voltage divider comprising resistors 4130 and 4131 is coupled to the output signal, i.e. to the output of the differential amplifier by coupling to its positive output voltage Vop and its negative output voltage Von respectively. Parallel to the resistors 4130, 4131 of this voltage divider is a capacitor 4140 and 4141 respectively. As the resistors and transistor 4121 comprise a parasitic capacitance to ground thus forming an unwanted RC low-pass filter, the capacitors parallel to the resistors enables Vcm_sense to react quickly to changes in the common mode voltage at vop and von respectively. The resistors and the capacitors respectively are of equal value, for example of 10 kΩ resistance and 100 femto Farad capacitance. As the values of the resistors and the capacitors are equal, the center voltage Vcm_sense of this second voltage divider is the common mode portion of the amplifier's output signal. The center voltage Vcm_sense is coupled as control voltage to thick NMOS transistor 4121. The current mirror comprising PMOS transistors 4150 to 4153 is tapped at the gate of transistor 4152 thus providing control voltage vbp_d, which is coupled to differential amplifier as shown in FIG. 4 a.

Metaphorically speaking the circuit of FIG. 4 b provides a calibration voltage vbp_d, which ensures the common mode output of the differential amplifier remains at half the supply voltage.

As indicated in the drawing each branch of the current mirror takes 100 microampere as input current, which is discharged via transistor 4160 as a 200 microampere current.

FIG. 5 depicts a first exemplary embodiment 500 of a circuit providing active output impedance.

In the exemplary disclosed circuit the DAC 120 provides an output current. As the active output impedance line driver as disclosed here requires a voltage as input signal the output current provided by the DAC must be transformed into a voltage to be fed into the active output impedance line driver. The current-to-voltage converter 510 as shown in FIG. 5 for example can do this transformation.

The current-to-voltage converter comprises first and second single ended amplifiers 511, 512, each having a resistor 513, 514 parallel to a capacitor 515, 516 in its feedback path. Exemplary values of these elements may be 1 kΩ for resistors 513, 512 and 115 femto Farad for capacitors 515, 516. Furthermore amplifiers 511 and 512 are coupled to voltage source 517 providing a direct voltage Vcm, which corresponds to the wanted common mode voltage of the output signal of the driver. Converter 510 provides a voltage at its output as the input current travels through the resistors and appropriately gets converted into an output voltage by action of the amplifier.

Note that any current-to-voltage topology may be used for converting the current as output by DAC 120 to a voltage as input for the active impedance output line driver 520. Furthermore and in case that the DAC is voltage mode the active impedance output line driver can be coupled directly to the DAC.

The exemplified active impedance output line driver 520 can be seen as a pseudo-differential line driver. The driver 520 comprises two portions of identical topology, wherein one driver portion outputs the positive portion of the output signal and the other portion outputs the negative portion of the output signal.

Each portion of driver 520 comprises a first resistor Rvi 521, 522 coupled to the negative input of a differential amplifier 523, 524, which can be of any topology. In the negative feedback path of the amplifiers that is from the positive output to its negative input a resistor Rfb 525, 526 is arranged parallel to a capacitor Cfb 527, 528. As mentioned above the negative feedback paths around the differential amplifiers may form a single-pole low-pass filter of a frequency around 80 MHz, when using the optional 17-level DAC as mentioned above.

The positive feedback path, that is from the negative output of the amplifier to the its positive input terminal, is shorted and coupled via resistor Rt 529, 530 to a direct voltage source Vcm corresponding to the common mode voltage of the output signal.

Exemplary values for the passive components may be 1.5 kΩ for resistors 521, 522 and 9 kΩ for feedback resistors 525, 526 and 16.6Ω for resistor 529, 530 and a capacitance of 200 femto Farad for the feedback capacitors 527, 528.

For this topology the output impedance Rout for one portion of the active output impedance line driver as shown in the drawing is given as

${Rout} = {{Rt} \cdot \frac{{Rvi} + {Rfb}}{{Rvi} + {Rt}}}$

and the output voltage Vout is given as

${Vout} = {{Vsig} \cdot \frac{{Rt} - {Rfb}}{{Rvi} + {Rt}} \cdot \frac{Rcable}{{Rcable} + {{Rt}\left( \frac{{Rvi} + {Rfb}}{{Rvi} + {Rt}} \right)}}}$

Metaphorically speaking each of the two driver portions generates one half of the maximum output voltage required by 10BT protocol. That is the portion outputting the positive signal portion Vop generates a voltage above the common mode portion of the output signal.

This topology allows the circuit to be supplied with a supply voltage of less than the required output signal voltage defined for 10BT protocol. Accordingly amplifiers 523, 524 can be supplied with 3.3 Volts, which is less than maximum signal swing of 5 Volts defined for 10BT.

The topology of the optional 17-level DAC in combination with the feedback paths of the amplifiers comprised in the line drivers thus discloses a 17-level digital-to-analog converter (DAC) followed by a single-pole low-pass filter for producing an analog signal corresponding to a digital symbol, which the DAC receives as input. This combination can be used to process signals for 1000BT and 100BT and 10BT Ethernet signals, wherein each signal matches the requirements set forth in the 802.3 IEEE specification. In particular an integrated circuit (IC) is disclosed comprising a 17-level digital-to-analog converter (DAC) receiving digital symbols and at least one clock signal and outputting corresponding analog signals, and wherein the output of the DAC is switchable coupled to a voltage mode line driver for producing 1000BT and 100BT Ethernet signals and to an active output impedance line driver for producing 10BT Ethernet, wherein the feedback paths of the line drivers each comprises a single-pole low-pass filter.

FIG. 6 depicts an alternative implementation 600 of a pseudo differential active output impedance line driver, which may replace the line driver 520 as illustrated in FIG. 5. Similarly this implementation of an active output impedance line driver comprises two portions of identical topology, the upper portion providing the positive output signal Vop and the lower portion providing the negative output signal Von.

Each portion comprises an input resistor R1 610, 611 coupled to the negative input terminal of a single ended amplifier 620, 621 respectively. The negative feedback path of each amplifier comprises a resistor R2 630, 631 and an optional capacitor 640, 641. The output of amplifiers 620, 621 is coupled via a termination resistor Rt 650, 651 to the output pads, such that the termination resistor is in series with the amplifier 620, 621 respectively. The positive input of amplifiers is coupled via a resistor R3 to a direct voltage source Vcm, which thus provides a reference potential to the amplifiers 620, 621, such that a common mode voltage is provided to the output signals Vop and Von.

This reference voltage Vcm is also fed forward via resistor R4 670, 671 to the output pads of the driver, wherein optional capacitors 680, 681 may be arranged in parallel to resistors R4.

Example values for the elements may be 1.5 kΩ for resistors R1, 12 kΩ for resistors R2, 1.5 kΩ for resistors R3, 13.5 kΩ for resistors R4 and 5Ω for Rt. Capacitors Cfb may have 200 femto Farad and capacitors 681 may have also 200 femto Farad. Although resistor Rt is in series with the amplifiers the output of the line driver still can be seen as an active output impedance, because the value of Rt is at least one order of magnitude smaller than the output impedance.

From the above illustrated examples it is apparent to those skilled in the art that any line driver providing active output impedance may be used in the line driver circuit.

While the invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed. 

What is claimed is:
 1. A line driver, comprising: a multiplexer, having first and second output ports, configured to multiplex an analog signal to provide a first signal or a second signal in response to a negotiated transmission speed; a first line driver, coupled to the first output port, configured to produce a first Ethernet signal, having a first speed, in response to the first signal; and a second line driver, coupled to the second output port, configured to produce a second Ethernet signal, having a second speed, in response to the second signal.
 2. The line driver of claim 1, wherein the first line driver comprises a voltage mode line driver, and wherein the second line driver comprises an active output impedance line driver.
 3. The line driver of claim 2, wherein an output stage of the voltage mode line driver is coupled to a calibration voltage that is configured to calibrate a common mode voltage of the voltage mode line driver, and wherein the output of the voltage mode line driver and a supply voltage are fed into a calibration circuit configured to produce the calibration voltage that is approximately half of the supply voltage.
 4. The line driver of claim 2, wherein the voltage mode line driver comprises a first single-pole low-pass filter in a feedback path of the voltage mode line driver, and wherein the active output impedance line driver comprises an operational amplifier having a second single-pole low-pass filter in a feedback path of the operational amplifier.
 5. The line driver of claim 4, wherein the first single-pole low-pass filter comprises an adjustable capacitor that is configured to adjust a frequency of the first single-pole low-pass filter, and wherein a frequency of the second single-pole low-pass filter is at a frequency of approximately 80 MHz.
 6. The line driver of claim 1, wherein the first Ethernet signal comprises a 100BT or a 1000BT Ethernet signal, and wherein the second Ethernet signal comprises a 10BT Ethernet signal.
 7. The line driver of claim 1, wherein the second line driver is further configured to be inactive when the first line driver is configured to produce the first Ethernet signal, and wherein the first line driver is further configured to be inactive when the second line driver is configured to produce the second Ethernet signal.
 8. The line driver of claim 1, wherein the first line driver is further configured to receive the first signal being clocked by a first clock signal corresponding to 100BT or 1000BT wired Ethernet signals, and wherein the second line driver is further configured to receive the second signal being clocked by a second clock signal corresponding to a 10BT wired Ethernet signal.
 9. The line driver of claim 1, further comprising: a connection pad configured to output the first or the second Ethernet signals to a transmission line, wherein the first line driver is further configured to exhibit a first impedance to the transmission line that is larger than a characteristic impedance of the transmission line when the first line driver is configured to be inactive, and wherein the second line driver is further configured to exhibit a second impedance to the transmission line that is larger than the characteristic impedance when the second line driver is configured to be inactive.
 10. A method for producing Ethernet signals, comprising: multiplexing an analog signal by a line driver to provide a first signal or a second signal in response to a negotiated transmission speed; and producing, by the line driver, a first Ethernet signal, having a first speed, in response to the first signal or a second Ethernet signal, having a second speed, in response to the second signal.
 11. The method of claim 10, further comprising: converting a plurality of digital symbols to the analog signal.
 12. The method of claim 11, wherein the second signal comprises a current-mode signal, and wherein producing comprises: converting the second signal from a current-mode signal to a voltage mode signal; and producing the second Ethernet signal in response to the voltage mode signal.
 13. The method of claim 10, wherein producing comprises: introducing an intermediate step in a rising edge and a falling edge of the analog signal when the negotiated transmission speed comprises a 100BT mode.
 14. The method of claim 10, wherein the first speed corresponds to a 10BT mode, and wherein the second speed corresponds to a 100BT mode or a 1000BT mode, further comprising: setting the negotiated transmission speed to the 10BT mode to produce the second Ethernet signal upon power up of the line driver; changing the negotiated transmission speed to the 100BT mode or the 1000BT mode during an ongoing transmission; ceasing production of the second Ethernet signal in response to the changing; and commencing production of the first Ethernet signal in response to the changing.
 15. The method of claim 10, further comprising: filtering the first signal using a first single-pole low-pass filter of adjustable frequency in the line driver, and filtering the second signal using a second single-pole low-pass filter in the line driver.
 16. An integrated circuit, comprising: a digital-to-analog converter (DAC) configured to output an analog signal; a multiplexer configured to multiplex the analog signal to provide a first signal or a second signal in response to a negotiated transmission speed; a first line driver, coupled to the multiplexer, configured to produce a first Ethernet signal, having a first speed, in response to the first signal, the first line driver comprising a first feedback path; and a second line driver, coupled to the multiplexer, configured to produce a second Ethernet signal, having a second speed, in response to the second signal, the second line driver comprising a second feedback path; wherein the first and the second feedback paths comprise a first and a second single-pole low-pass filter, respectively.
 17. The integrated circuit of claim 16, wherein the DAC comprises a multi-level DAC.
 18. The integrated circuit of claim 16, wherein the second line driver is further configured to be inactive when the first line driver is configured to produce the first Ethernet signal, and wherein the first line driver is further configured to inactive when the second line driver is configured to produce the second Ethernet signal.
 19. The integrated circuit of claim 18, further comprising: a connection pad configured to output the first or the second Ethernet signals to a transmission line; wherein the first line driver is further configured to exhibit a first impedance to the transmission line that is larger than a characteristic impedance of the transmission line when the first line driver is configured to be inactive, and wherein the second line driver is further configured to exhibit a second impedance to the transmission line that is larger than the characteristic impedance of the transmission line when the second line driver is configured to be inactive.
 20. The integrated circuit of claim 16, wherein the first line driver is further configured to receive the first signal clocked by a first clock signal corresponding to 100BT or 1000BT Ethernet signals, and wherein the second line driver is further configured to receive the second signal clocked by a second clock signal corresponding to a 10BT Ethernet signal. 